The present invention relates to a decision circuit and particularly to control of decision timing of a decision circuit to be used for a demodulating operation of multi-level QAM (Quadrature Amplitude Modulation) systems.
In the field of digitl radio communication, development of multi-level demodulation systems such as 16-QAM, 64-QAM and 256-QAM, etc. is continued in view of improving efficiency of frequency application. With such development, the decision error rate of a received signal rises and therefore a decision circuit must be much improved.
In the existing decision circuit, the I (inphase) signal and Q (quadrature) signal obtained by demodulating the received signal are respectively amplified and then decided by an A/D converter functioning as a decision circuit. Operation is dependent upon a clock signal which is recovered from the demodulated signal by the clock recovery circuit. The demodulated signal is then and finally converted into decision data of in I signal and Q signal. In this case, 2 bits in the case of 16-level QAM, or 3 bits in the case of 64-level QAM, or 4 bits in the case of 256-level QAM become the decision data bit output from the A/D converters for the signal channel of the I or Q signal. These A/D converters allow input of the clock signal through a phase shifter for adjustment of decision timing but this timing is initially set by manual operation for the I signal and Q signal depending on accuracy of decision of the test signal.
As explained above, since the decision circuit of the prior art has fixed the decision timing with the initially set clock phase shifter, the clock timing error becomes large and decision data error is generated even with a little fluctuation of power supply, temperature change of the receiver or a deviation between the demodulated signal and recovered clock due to deterioration of line condition. This is because the sampling time is comparatively narrow in the multi-level QAM system where band limitation is effectuated by a roll-off filter. Accordingly, the prior art has a serious problem that line quality is deteriorated.